Problem: Decode Ways

Question

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example, Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

Solution

TODO

Code

class Solution {
    public int numDecodings(String s) {
        if(s.length() == 0 || s.charAt(0) == '0') return 0;
        int[] decodeWays = new int[s.length()+1];
        decodeWays[0] = 1;
        decodeWays[1] = 1;
        for(int i=1; i<s.length(); i++) {
            int ways = 0;
            if(s.charAt(i) != '0') ways += decodeWays[i];
            int val = Integer.valueOf(s.substring(i-1, i+1));
            if(val <= 26 && val >= 10) ways += decodeWays[i-1];
            if(ways == 0) return 0;
            decodeWays[i+1] = ways;
        }
        
        return decodeWays[s.length()];
    }
}

Performance